The present invention relates to a secondary heat transfer circuit for a nuclear reactor cooled by a liquid metal such as sodium, as well as to a steam generator particularly suitable for such a circuit.
In the present state of the art, fast neutron nuclear power stations conventionally comprise a primary circuit in which the sodium extracts the heat produced by the fuel elements of the core, in order to transfer it to a heat exchanger, in which it transfers its heat to sodium contained in a secondary heat transfer circuit. The hot sodium leaving the exchanger in said secondary circuit transfers the heat to a steam generator, where it gives up its heat to the pressurized water contained in a tertiary heat transfer circuit supplying steam to a turbine in order to produce electricity.
According to a so-called integrated reactor design, the primary heat transfer circuit is completely integrated into a vessel containing both the core of the reactor, the exchangers and the circulating pumps of the primary circuit, together with the liquid sodium contained in said circuit. As there are three or four heat exchangers the secondary heat transfer circuit outside the reactor vessel is usually divided into three or four identical loops, which also applies with respect to the tertiary heat transfer circuit.
In existing fast neutron nuclear power stations, the existence of a secondary heat transfer circuit is justified by the need of very safely confining the radioactive primary sodium and because it is necessary to protect the primary circuit from the possible repercussions of a leak from the heat exchange surface of the steam generator. Thus, in such a case, the pressurized steam or water would come into contact with the sodium and the chemical reaction produced is highly exothermic, releasing dangerous and corrosive reaction products (caustic soda and hydrogen). It is therefore necessary to protect the core, i.e. the primary circuit, from the possible repercussions of this sodium--water reaction (overpressure and pollution by sodium).
FIG. 1 shows the conventional construction of a secondary cooling loop of a fast neutron nuclear reactor. In the embodiment shown in continuous line form, this loop comprises a heat exchanger 4, whilst in the embodiment shown in broken line form, a further heat exchanger. The exchangers 4 are placed in vessel 2, sealed by a slab 3 and containing the reactor core 7 and the complete primary heat transfer circuit. More particularly, vessel 2 is filled with liquid sodium 5. The outlet from each exchanger 4 is connected to a steam generator 6 by an outgoing pipe 8. In the upper part of the steam generator, there is an argon pocket 6a, which defines the free sodium level N. The outlet of steam generator 6 is connected by a pipe 10 to the suction opening of a circulating pump 12, whereof each delivery opening is connected by a pipe 14 to the intake of an exchanger 4.
In such a conventional secondary cooling loop, the passage of the pump shaft is sealed by means of a mechanical packing, which is in contact with a neutral gas such as argon placed between the sodium and the packing. For this purpose, the free sodium level N1 is surmounted by an argon pocket 12a. Moreover, the pump rotor is placed in an expansion tank 18, whose size is adequate to absorb, without immersing the packing, all possible increases in the sodium volume in the secondary loop (by thermal expansion). Finally, in order to avoid any risk of the lining being immersed, which might occur in the case of an argon loop surmounting the sodium in tank 18, the latter is placed at the highest point of the circuit.
A pipe 20 for topping up with sodium connects a storage tank 24 placed in the lower part of the installation to the upper part of tank 18. This pipe 20 is equipped with a circulating pump 19 and a sodium purification system 21. It makes it possible to compensate any leak of the drain valve V2, which is positioned in the drain pipe 38 of pipe 10. Conversely, any inopportune rise in the free sodium level N1 of tank 18 is compensated by an overflow pipe 22, which connects tank 18 to pipe 36 below drain valves V1, located in the drain pipes 36 of pipes 8.
The argon pressure in tank 18 is controlled by an inlet tube 23, whilst the free sodium level N2 in tank 24 is surmounted by an argon covering 24a, whose pressure is controlled by a tube 26. Moreover, the argon coverings or pockets 6a, 12a of the generator and the pump communicate by both pipe 25 for balancing levels N and N1 and the pressures.
As is also illustrated in FIG. 1, tank 34 is also used for recovering any products resulting from a sodium--water reaction in the case of a leak in the steam generator 6. To this end, a pipe 30 connects the lower part of steam generator 6 to the upper part of tank 24. This pipe is normally sealed by large diameter bursting disks 28, which burst under the effect of overpressures due to the sodium--water reaction and thus permit the decompression of the secondary loop. In this case, tank 24 ensures the separation of the liquid and gaseous products resulting from the sodium--water reaction. However, this function can also be assured by a second separator 32, e.g. of the cyclone type, communicating with tank 24 by a pipe 31 and provided with a stack 34 for discharging to the atmosphere the gaseous products (hydrogen, argon, steam).
In addition to the means making it possible to recover the products of the sodium--water reaction which could take place in the steam generator, means are provided for damping shocks in the secondary circuit upstream and downstream of the steam generator, in order that they are not transmitted to the heat exchangers 4. Upstream of the exchange zone of generator 6, said means are constituted by the argon pocket 6a formed within the generator and in the upper part thereof. This argon pocket 6a then fulfils the function of an upstream buffer tank. Downstream of the generator, the downstream buffer tank coincides with the expansion tank 18 of the pump having the argon covering 12a. The prior art secondary loop described hereinbefore has a number of disadvantages mainly resulting from the need of placing the pump in the upper part of the installation and of positioning the pump rotor in a large-size expansion tank 18.
The raised position of the pump places it under poor hydraulic suction conditions, which make it necessary to adopt a low rotation speed and consequently a large diameter wheel and a slow drive motor to avoid cavitation. The assembly has large dimensions and is costly, because as is known the price of a pump increases with the square of its diameter. Parallel to this, the expansion tank surrounding the pump is heavy and expensive. Moreover, the combination of the weight of the pump--tank assembly and its arrangement in the upper part of the installation involves the use of a large support structure, particularly to obviate possible seismic effects, which tend to increase on increasing elevation. Moreover, bearing in mind the location of the different elements of the loop and the need to be able to empty these by gravity, the piping equipping the installation is particularly long and cumbersome. Finally, the arrangement of the expansion tank is such that it forms a system of vessels communicating with the steam generator. In the case of a sodium--water reaction in the latter, there are then large amplitude oscillations, which the expansion tank finds it difficult to contain.
In order to obviate these disadvantages, European Patent Application No. 0,014,662 proposes eliminating the expansion tank surrounding the pump and placing the latter in the lower part of the installation, just above the storage tank. This solution is made possible by introducing the storage tank into the active part of the secondary loop and by making it simultaneously serve as a downstream anti-water hammer tank and as an expansion tank. However, it makes it necessary to balance the sodium pressure throughout the secondary loop by placing under an adequate pressure the argon covering over the sodium in the storage tank. In practice, this pressure must be approximately 3 bars relative.
This known solution has the advantage of placing the pump under satisfactory hydraulic operating conditions, but the use of a pressurized storage tank, equipped with immersing pipes causes a significant increase in the risks resulting from the use of sodium as the cooling fluid. In particular, in the case of a leak in the circuit, the leakage flows are significantly increased and with them the risks of ignition of the atomized sodium. Among the other disadvantages of such a solution, reference is made to the impossibility of insulating the storage tank from the remainder of the circuit and the existence of transient phenomena of a difficult nature as a result of the reversal of the leakage flow direction in the pump. Moreover, it is certain that the multiplication of the functions fulfilled by the storage tank makes it particularly difficult to control thermohydraulic phenomena, which occur in said tank and the corresponding thermomechanical loading.